Removal and recovery of sulfur from a gas stream containing hydrogen sulfide

ABSTRACT

H2S is removed from a gas stream and sulfur is produced by the steps of: (1) sequentially scrubbing the gas stream with a first recycle water stream containing NH4OH and then with a second recycle water stream which is sbustantially free of NH4OH to produce a treated gas stream which is reduced in H2S content and is substantially free of NH3 and a bottom water stream containing NH4HS; (2) catalytically treating the bottom water stream with an air stream to produce an effluent stream containing ammonium polysulfide; (3) treating the ammonium polysulfide-containing stream to recover sulfur and to produce a water stream which is substantially free of NH4OH and a water stream containing NH4OH; and, (4) separately recycling at least a portion of these last two streams to the scrubbing step.

United States Patent 191 Hamblin 75 Inventor: Robert J. J. Haniblin,Deerfield, 111.

[73] Assignee: Universal Oil Products Company,

Des Plaines, ill.

[22] Filed: July 19, 1971 [21] Appl. No.: 163,844

Related US. Application Data [63] Continuation-impart of Ser. No.802,356, Feb. 26,

1969, Pat. No. 3,594,125.

[52] US. Cl ..423/573, 423/220 [5 l] Int. Cl. ..C0lb 17/04 [58] Field ofSearch ..23/l8l, 224, 225; 210/63 [56] References Cited UNITED STATESPATENTS 3,365,374 1/1968 Short et al. .23/181 X 3,457,046 7/1969Hoekstra ..23/224 [451 Apr. 17, 1973 3,627,470 12/1971 Hamblin ..23/224Primary ExaminerOscar R. Vertiz Assistant ExaminerGeorge O. PetersAttorney-James R. Hoatson, Jr. et al.

[5 7] ABSTRACT 1-1 8 is removed from a gas stream and sulfur is producedby the steps of: (l) sequentially scrubbing the gas stream with a firstrecycle water stream containing Nl-LOH and then with a second recyclewater stream which is sbustantially free of Nl-LOH to produce a treatedgas stream which is reduced in H 8 content and is substantially free ofNH and a bottom water stream containing NHJ-IS; (2) catalyticallytreating the bottom water stream with an air stream to produce aneffluent stream containing ammonium polysulfide; (3) treating theammonium polysulfidecontaining stream to recover sulfur and to produce awater stream which is substantially free of NH OH and a water streamcontaining NH OH; and, (4) separately recycling at least a portion ofthese last two streams to the scrubbing step.

11 Claims, 1 Drawing Figure REMOVAL AND RECOVERY OF SULFUR FROM A GASSTREAM CONTAINING HYDROGEN SULFIDE CROSS-REFERENCE TO RELATEDAPPLICATIONS This application is a continuation-in-part of my prior,copending application, Ser. No. 802,356, which was filed on Feb. 26,1969, now US. Pat. No. 3,594,125.

The subject of the present invention is a novel process for convenientlyremoving H 8 from a gas stream sequentially scrubbing with an ammoniacalwater stream and with an ammonia-free water stream without comtaminatingthe treated gas stream with NH;, and for recovering elemental sulfurfrom the resulting rich absorbent stream with continuous regeneration ofthe two scrubbing streams. More precisely, the present invention isbased on my finding of a convenient and simple method for treating awater stream containing NH HS to produce elemental sulfur and two waterstreams: one containing Nl-LOH and one substantially free of NH OH,coupled with my recognition that a gas scrubbing step can beconveniently and simply interconnected with this treating method bymeans of these two water streams to enable the continuous scrubbing of agas stream containing H 8 without contaminating the treated gas streamwith substantial amounts of NH The removal of H 8 from a gas stream is aproblem that has long confronted and challenged workers in many diverseindustries. One example is in the natural gas industry where the H 8content of certain gas streams recovered from natural gas deposits inmany areas of the world is often too high for commercial acceptance.Another example is in the manufactured gas industry of the coke-makingindustry where coal gas containing unacceptable amounts of H 8 iscommonly produced by the destructive distillation of bituminous coalhaving a high sulfur content. Yet another example is found in themanufacture of water gas or synthesis gas where it is not unusual toproduce gas streams containing H S by passing steam over a bed ofincandescent coke or coal containing a minor amount of sulfur.

More frequently, this problem is encountered in the petroleum refiningindustry because the principal raw material used, crude oil, typicallycontains a minor amount of sulfur principally in the form of organicsulfur compounds. During the course of the many processes to which thecrude oil or fractions thereof are subjected, one or more gas streamscontaining H 8 are quite commonly produced. For example, in many casesone of the product streams from a hydrocarbon conversion process is agas stream containing l-I,S in admixture with light normally gaseoushydrocarbons mainly, C -C and/or with hydrogen. As is well known in theart, the presence of H 8 in these refinery gas streams can cause anumber of detrimental problems in subsequent processing steps such as:corrosion of process equipment, deterioration and deactivation ofcatalysts, undesired side reactions, increases in process pressurerequirements, increase in gas compressor capacity, etc.

Regardless of the source of the gas stream containing H 8, the problemof removing H S therefrom has been solved in a number of different wayswhich generally involve one or more of the following techniques:selective absorption with a wide variety of absorbents, adsorption by asuitable adsorbent, selective reaction with a reagent which produces aneasily separable product, etc. The details of these techniques are wellknown to those skilled in the art. One old and well known solution tothis H 8 removal problem involves scrubbing the gas stream with anammoniacal aqueous solution. For example, in Germany the Perox process,which uses ammonia scrubbing, has been widely used for coal gaspurification. Despite the considerable amount of effort that has beendevoted to developing an acceptable solution to this problem involvingscrubbing with an ammoniacal solution, the use of ammoniacal scrubbinghas not been universally accepted in the gas treating art as thepreferred method for removing H 8 from a gas stream primarily because ofa number of operational difficulties associated with its implementation.One difficulty'involves the high partial pressure of ammonia whichgenerally requires that the scrubbing step be conducted with relativelydilute ammonia solutions or at relatively high pressures. The use ofdilute scrubbing solutions in turn quite commonly forces the addition ofa separate water wash step after the ammonia scrubbing step in order toremove ammonia from the treated gas stream. In addition, the use ofdilute scrubbing solutions typically increases substantially theregeneration costs where the regeneration step is conducted at aconsiderably higher temperature than the scrubbing step, although someof this heat load can be.

' recovered by a suitable heat exchanging procedure.

Another difficulty is associated with the regeneration of the richabsorbent solution withdrawn from the scrubbing step. In order tominimize the requirements of the scrubbing step for water and ammonia,it is necessary to remove sulfide from this rich absorbent solution.Several regeneration procedures have been proposed but they typicallyhave involved the use of soluble catalysts such as hydroquinone and havehad problems such as contamination of the sulfur product with thecatalyst, excessive formation of side products such as ammonium sulfateand thiosulfate and loss of scrubbing solution and catalyst during theperiodic purges that are generally required to remove side products fromthe system. Other difficulties have been associated with the recovery ofthe elemental sulfur from the regeneration step where it has beencustomary to form a froth of sulfur which then must be skimmed off andfiltered. In short, it is clear that there are a significant number oftechnical problems associated with the prior art methods for removing HS from a gas stream by the method of scrubbing with an ammoniacalsolution.

As a result of my investigations of method of treating water streamscontaining NHJ'IS, lhave now formultaed a new approach to the use ofammonia scrubbing for the solution of this H 8 removal problem whichapproach overcomes many of the difficulties experienced in the priorart. The basic concept of my approach involves the production in theregeneration section via a simple, economic procedure of two separaterecycle water streams, one containing NH OH and one substantially freeof Nl-l,,OH and the interconnection of the ll S-scrubbing step and theregeneration section by means of these two recycle streams in a matterthat minimizes the amount of ammonia carried out of the system via thetreated gas stream. More specifically, my solution to the probleminvolves the scrubbing of the sour gas stream with an ammoniacalsolution and with a solution free of ammonia to form a rich absorbentsolution containing NH,HS and the regeneration of this solution by firstsubjecting this stream to contact with a solid catalyst at conditionsresulting in the formation of ammonium polysulfide followed by thesubsequent decomposition of the ammonium polysulfidecontaining stream torecover sulfur and to produce the two recycle water streams. Some of theadvantages associated with my solution of this H 8 removal problem are:(l) the sulfur recovered is not contaminated with detrimental salts; (2)the ammonia loss from the system is held to low levels; (3) thescrubbing solution is not highly corrosive and metallurgy problems areminimized; (4) the amount of catalyst lost during the operation of theprocess is inconsequential; (5) a minimum amount of water is evaporatedin the regenerationsection; (6) the ultimate yield of elemental sulfuris quite high; and (7) the requirement of the process for water isminimized through the use of recycle streams.

It is, accordingly, a principal object of the present invention toprovide a new process for removing 1-1 8 from a gas stream with anammoniacal solution. Another object is to provide a simple technique forremoving H 8 from a gas stream with an ammoniacal solution withsubsequent recovery of elemental sulfur.

Yet another object is to provide a process for scrubbing contact betweena gas stream and a water stream.

Similarly, a first recycle water stream containing NI-LOH is introducedinto the middle region of the first scrubbing zone and a second recyclewater stream which is substantially free of ammonia and sulfide isintroduced into the first scrubbing zone at a point above the point ofintroduction of the first recycle stream. The first scrubbing zone ismaintained under countercurrent liquid-gas contact conditions selectedto produce agaseous overhead stream which is substantially reduced inH,S content and is substantially free of NH:, and an aqueous bottomstream containing NH HS and typically some NH OH. in the next step, theaqueous bottom stream from the scrubbing zone, an air stream, and athird recycle water stream containing Nl-LHS, (NH ),S,O,, and NH 0H arecontacted with a solid catalyst at oxidizing conditions selected to forman effluent stream containing ammonium polysulfide, 4)2S2O3NH4OH, H2O, Nand typically some unreacted NHJ-IS. The effluent stream from thisoxidation step is then separated into the gas stream containing N,, H,O,H,S, and N11,, and a liquid stream containing ammonium polysulfide, NHOH, H 0, (NH ),S,O and typically some Nl-LHS. Following this gas-liquidamount of (NH ),S,O which stream is substantially free of ammonia andsulfide. A first portion of the water stream from the sulfur separationstep is then used in a second scrubbing zone to remove H 5 and NH fromthe gas stream produced in the gas-liquid separation step to form anitrogen-rich overhead gas stream and an aqueous bottom streamcontaining (NH4)zS2O NH4OH and In the next step, the'overhead vaporstream from the polysulfide decomposition step is introduced into athird scrubbing zone which is operated under reflux conditions selectedto form a substantially sulfide-free overhead vapor stream containingNH;, and H 0 and an aqueous bottom stream containing NHJ-IS and NH OH.Thereafter, at least a portion of the overhead stream from the thirdscrubbing zone is condensed to form a substantially sultide-freeammoniacal water stream, a first portion of which is returned to thethird scrubbing zone as reflux. The bottom streams from the second andthird scurbbing zones are then combined to form the third recycle waterstream which is then passed to the oxidation step. In addition, a secondportion of the water stream from the sulfur separation step is recoveredas the second recycle water stream and passed to the first scrubbingzone. The first recycle water stream is then recovered as a secondportion of the ammoniacal water stream formed in the overhead vaporcondensing step, and the resulting stream is passed to the firstscrubbing zone. And in the final step, a third portion of the waterstream from the sulfur separation step is withdrawn from the process inorder to remove the net amount of (NHUiSgOn formed therein.

Other embodiments and objects of the present invention encompass detailsabout particular input streams, output streams, and the mechanicsassociated with each of the essential and preferred steps thereof. Theseare hereinafter disclosed in the following detailed discussion of thesefacets of the present invention.

The invention will be further described with reference to the attacheddrawing which is a schematic outline of the process under discussion.The attached drawing is merely intended as a general representation of apreferred flow scheme with no intent to give details about vessels,heaters, condensers, pumps, compressors, valves, process controlequipment, etc., except where a knowledge of these devices is essentialto the understanding of the present invention or would not beself-evident to one skilled in the art.

Referring now to the attached drawing, a gas stream containing H,Senters the process through line 1 and is charged to the lower region ofthe first scrubbing zone, zone 2. The gas stream may be derived from anumber of difi'erent sources and may be a coal gas, or oil gas, a watergas, a natural gas, a refinery gas and the like gas streams. In order toavoid confusion, some of the various types of gas streams which can becharged to this process are defined as follows: (1) coal gas is amixture of gases produced by the destructive distillation of coal;

(2) an oil gas is a gas derived from petroleum by the interaction of oilvapors and steam at high temperatures; (3) a water gas, or a synthesisgas, as it is sometimes called, is a gas made by decomposing steam bypassing it over a bed of incandescent coke or coal, and in some cases itis made by the high temperature reduction of steam with natural gases orsimilar hydrocarbons; (4) a natural gas is a mixture of low molecularweight paraffin hydrocarbons typically C -C and, (5) a refinery gas is amixture of low molecular weight hydrocarbon gases and/or hydrogenproduced in converting and distilling hydrocarbons, In all cases, thegas stream to be treated by the present invention will contain H S in anamount ranging from about 0.01 mole percent up to about 50 mole percentor more. Typically, the amount of H 8 contained in this gas stream willbe about 1 to about mole percent. In addition, in some cases the gasstream may contain ammonia, and in this case the present invention willalso substantially remove and recover the ammonia from the gas stream.

Zone 2 is preferably a vertically positioned tower containing suitablemeans for achieving intimate contact between a gas stream and a liquidstream. Suitable contacting means are trays, plates, baffles or anysuitable packing material such as Raschig rings. Zone 2 is convenientlydivided into three regions; a bottom region where the input gas streamenters; a middle region where a first recycle water stream enters, and atop region where a second recycle water stream is injected. The inputgas stream is charged via line 1 to the bottom region of zone 2 where itintimately contacts a descending water stream which is a mixture of thefirst recycle water stream, which is injected via line 26, and a secondrecycle water stream, which enters zone 2 by means of line 21. Asindicated hereinbefore, it is an essential feature of the presentinvention that the first recycle water stream contains NI-I OI-I andthat the second recycle water stream is substantially free of NH OH. Theamount of NI-LOH contained in the first recycle water stream may rangefrom about 1 wt. percent up to about 50 wt. percent. In general, it isnecessary to inject sufficient NI-l via line 26 to provide at least 1mole of NH per mole of H 8 entering zone 2 via line 1, and, morepreferably, about 1 to about 5 or more moles of NH per mole of H S.Accordingly, for a particular H 8 loading on zone 2, the NI-LOI-Iconcentration in the first recycle water stream, its rate of circulationand operating conditions in zone 2 are selected to provide a Nil /H 5mole ratio within the specified range and to reduce the l-l,S content oftreated gas stream to the desired low level which typically is of theorder of 10 to 500 vol. ppm.

In the upper region of zone 2 the partially scrubbed gas stream iscountercurrently contacted with the second recycle water stream underconditions designed to remove substantially all volatilized NH, from thegas stream. This second recycle water stream is substantially free of NHtypically less than 300 wt. ppm. because it is recovered from the bottomstream from the polysulfide decomposition zone as is explainedhereinafter. The rate of circulation of this second recycle water streamis generally conveniently selected for a particular input gas stream andNI-I II-I S loading on the basis of a simple experiment designed todetermine the amount required to keep the treated gas stream withdrawnfrom the top of zone 2 substantially free of NH Generally, good resultsare obtained when the amount of the second recycle water stream is about1 to about 10 times the amount of the first recycle water stream in avolume basis, with a preferred value being selected from the range ofabout 1.511 to about 4: 1.

Regarding the scrubbing conditions utilized in zone 2, it is preferredto operate this zone at a relatively low temperature and relatively highpressure. Typcially, good results are obtained at a temperature of 50 F.to about 150 F. and a pressure ranging from about 1 to about 500atmospheres. For example, excellent results are obtained at atemperature of about F. and a pressure of about 10 atmospheres.

Following contact of the gas stream with the two recycle water streams,a treated gas stream is withdrawn from the upper region of zone 2 bymeans of line 3. Similarly, an aqueous bottom stream containing Nl-IJ-ISand typically some NH OI-l is withdrawn from the bottom region thereofvia line 4. Although it is not essential, in some cases where verydilute gas streams are being treated in zone 2, the bottom streamtherefrom may be advantageously further treated to concentrate theNI-LHS contained therein to yield a water stream containing about 3 to15 wt. percent as NI-I HS. Generally, this concentration step can beeasily effected by stripping NFL, and H S therefrom and redissolving theresulting gas stream in the required quantity of water or byrecirculating at least a portion of this bottom stream around the lowerregion of zone 2. However, in most cases the amount of NI-lJ-IScontained in the bottom stream is sufficient to allow the direct passageof it to treatment zone 6 as is shown in the drawing. This is especiallytrue when the amount of unreacted sulfide recycled to zone 6 via lines11 and 12 is sufficient to increase the concentration of sulfide in thecombined water stream charged to zone 6 to about 3 to 15 wt. percentthereof.

Accordingly, in the embodiment disclosed in the drawing,-the aqueousbottom stream from zone 2 is passed via line 4 to the junction of line 5with line 4 where it is commingled with an air stream, and the resultingmixture passed to treatment zone 6. The amount of oxygen contained inthis air stream is selected so that ammonium polysulfide is formedwithin zone 6. In order to effect the polysulfide formation in zone 6,the amount of oxygen injected into this zone must be carefully regulatedso that oxygen is reacted therein in an amount less than thestoichiometric amount required to oxidize all of the ammonium sulfidesalt charged to this zone to elemental sulfur. Since the stoichiometricamount of oxygen is 0.5 mole of oxygen per mole of sulfide, it isessential that the amount of oxygen charged to the treatment zone issufficient to react less than 0.5 mole of sulfide, and, preferably,about 0.25 to about 0.45 moles of oxygen per mole of sulfide salt. It isespecially preferred to operate with an amount of oxygen sufficient toreact about 0.4 mole of oxygen per mole of sulfide charged to this zone.Accordingly, the amount of oxygen, charged to zone 6 via lines 5 and 4,is selected such that sufficient unreacted sulfide remains available toform a water-soluble ammonium polysulfide with the elemental sulfurwhich is the product of the primary oxidation reaction. Since 1 mole ofsulfide will react with many atoms of sulfur (for ammonium polysulfide,it is typcially about four atoms of sulfur per mole of sulfide), it isgenerally only necessary that a small amount of sulfide remainunoxidized.

According to the present invention, the aqueous bottom stream from zone2 is passed to zone 6 wherein it is catalytically treated with oxygen atoxidizing conditions selected to produce an effluent stream containingammonium polysulfide, Nl-I OH, (NI-13 8,0 H O, N and typically someunreacted NHJ-IS. A feature of the present invention is the comminglingof the water stream feed to this oxidation step with a third recyclewater stream containing unreacted sulfide recovered from the effluentstream from treatment zone 6. This third recycle water stream may becommingled with the aqueous bottom stream from zone 2 prior ot its beingpassed into the treatment zone; on the other hand, this recycle waterstream can be injected into the treatment zone at a plurality ofinjection points spaced along the direction of flow of the bottom streamthrough the treatment zone as is shown in the attached drawing where thethird recycle water stream enters zone 6 via lines 11 and 12. Theprincipal advantage of this latter procedure is that the recycle streamacts as a quench stream for the exothermic reactions taking place withinthe treatment zone. Another advantage associated with use of the thirdrecycle water stream is that the concentration of NHJ-IS charged to thetreatment zone is increased. Since it has been determined that theselectivity of the oxidation reaction for elemental sulfur increaseswith the concentration of sulfide charged to the oxidation step, thepresence of sulfide in the recycle stream can be used to increase theselectivity for sulfur of the treatment zone. In fact, it is a preferredprocedure to use the third recycle water stream to maintain theconcentration of ammonium hydrosulfide in the combined water streamcharged to zone 6 at about 3 to about 15 wt. percent calculated aselemental sulfur.

The catalyst utilized in treatment zone 6 is any suitable solid catalystthat is capable of accelerating the oxidation of ammonium hydrosulfideto elemental sulfur. Two particularly preferred classes of catalyst forthis step are metallic sulfides, particularly iron group metallicsulfides, and metal phthalocyanines.

The preferred metallic sulfide catalyst is selected from the groupconsisting of the sulfides of nickel, cobalt, and iron, with nickelsulfide being especially preferred. Although it is possible to performthis oxidation step with a slurry of metallic sulfide particles, it ispreferred that the metallic sulfide be combined with a suitable carriermaterial. Examples of suitable carrier materials are: charcoals, such aswood charcoal, bone charcoal, etc., which charcoals may or may not beactivated prior to use; refractory inorganic oxides such as alumina,silica, zirconia, bauxite, etc.; activated carbons such as thosecommericallyavailable under trade names of Norit, Nuchar, and Darco andother similar carbon materials familiar to those skilled in the art. Inaddition, other natural or synthetic highly porous inorganic carriermaterials such as various forms of clay,

kieselguhr, etc., may be used if desired. The preferred carriermaterials for the metallic sulfide catalyst are alumina, particularlyalpha-, gamm-, and eta-alumina, and activated charcoal. Thus, nickelsulfide combined with alumina or nickel sulfide combined with activatedcarbon are particularly preferred catalysts for the oxidation step. Ingeneral, the metallic sulfide is preferably combined with the carriermaterial in amounts sufficient to result in a final composite containingabout 0.1 to about 30 or more wt. percent of the metallic component,calculated as the elemental metal. For the preferred nickel sulfidecatalyst, excellent results are obtained with about 1 to about 15 wt.percent nickel as nickel sulfide on an activated carbon support or on analumina support.

An especially preferred catalyst for use in the treatment zone 6 is ametal phthalocyanine compound combined with a suitable carrier material.Particularly preferred metal phthalocyanine compounds include those ofcobalt and vanadium. Other metal phthalocyanine compounds that may beused include those of iron, nickel, copper, molybdenum, manganese,tungsten, and the like. Moreover, any suitable derivative of the metalphthalocyanine may be employed including the sulfonated derivatives andthe carboxylated derivatives. Any of the carrier materials previouslymentioned in connection with the metallic sulfide catalysts can beutilized with the phthalocyanine compound; however, the preferredcarrier material is activated carbon. Hence, a particularly preferredcatalyst for use in the oxidation step comprises a cobalt or vanadiumphthalocyanine sulfonate combined with an activated carbon carriermaterial. Additional details as to alternative carrier materials,methods of preparation, and the preferred amounts of catalyticcomponents are given in the teachings of US. Pat. NO. 3,108,081 fo thesephthalocyanine catalysts.

Although the operation of zone 6 can be performed according to any ofthe methods taught in the art for simultaneously contacting a liquidstream and a gas stream with a solid catalyst, the preferred procedureinvolves a fixed bed of the solid catalyst disposed in the treatmentzone. The aqueous bottom stream from zone 2 is then passed therethroughin either upward, radial, or downward flow and the air stream is chargedin either concurrent or countercurrent flow relative to the waterstream. The preferred procedure is to operate downfiow with both streamsbeing charged in concurrent fashion. Because one of the products of thisoxidation step is elemental sulfur, there is a substantial catalystcontamination problem caused by the deposition of this elemental sulfuron the fixed bed of the catalyst. In order to. avoid sulfur despositionon the catalyst, it is necessary to operate so that the net sulfur madein this zone is reacted with excess sulfide to form a water-solubleammonium polysulfide.

Regarding the conditions utilized in treatment zone 6, it is preferredto utilize a temperature in the range of about to about 200 F, with atemperature of about to about F. yielding best results. The sulfideoxidation reaction is not too sensitive to pressure, and, accordingly,any pressure which maintains the water stream charged to zone 6substantially in the liquid phase may be utilized. In general, it ispreferred to operate at the lowest possible pressure which is sufficientto maintain the elemental sulfur in combination as the water-solubleammonium polysulfide, and although pressures of about I to about 30psig. may be used, a pressure of about 1 to about 20 psig. isparticularly preferred. Additionally, it is preferred to operate on thebasis of a combined stream liquid hourly space velocity which is definedas the volume charge rate per hour of the aqueous bottom stream fromzone 2 plus the third recycle water stream divided by a total volume ofthe catalyst bed. This parameter is preferably selected from the rangeof about 0.6 to about 20 hr.', with a value of about 1 to about hr.-giving best results.

An effluent stream is then withdrawn from zone 6 and found to containammonium polysulfide, Nl-LOI-I, (NH4)2S2O H2O, N in many cases unreactedN l-I HS, and typically unreacted 0 This stream is passed via line 7 togas-liquid separating zone 8 and therein separated into a gas streamcontaining N H 0, H 8, NI-I and typically unreacted O and a water streamcontaining ammonium polysulfide, NH OH, (NH S O and typcially someunreacted NHJ-IS. This separation step is preferably performed atapproximately the temperature and pressure maintained at the outlet fromthe oxidation step.

The water stream from separation zone 8 is then passed via line 13 topolysulfide decomposition zone 14. In this zone, the ammoniumpolysulfide is decomposed to yield NH H 8, and elemental sulfur.Although the polysulfide can be decomposed according to any of themethods taught in the art, the

preferred procedure involves subjecting it to conditions, including atemperaturein the range of about 200 F. to about 350 F. and a pressureof about 1 to about 75 psig., sufficient to form anoverhead vapor streamcontaining Nl-l H 8, H 0 which is withdrawn from zone 14 via line 15 andan aqueous bottom stream containing elemental sulfur and a minor amountof (NI-I S2O which is withdrawn via line 18. In many cases, it isadvantageous to accelerate the polysulfide decomposition reaction bystripping H 8 and NH from the polysulfide solution with the aid of asuitable inert gas such as steam, nitrogen, air, flue gas, etc. whichcan be injected into the bottom of the decomposition zone. Moreover,upflowing vapors may be generated in situ by supplying heat to thebottom of zone 14 by means such as a steam coil or reboiler in order toaccelerate the decomposition reaction.

When the temperature utilized in the bottom of decomposition zone 14 isless than the melting point of sulfur, the elemental sulfur will bepresent in the form of a slurry of solid particles in the aqueous bottomstream from zone 14. This slurry-containing bottom stream is thensubjected, in sulfur recovery zone 19, to any of the techniques taughtin the art for removing a solid from a liquid such as filtration,settling, centrifuging, etc., to remove the elemental sulfur therefromand to form a treated water stream containing a minor amount of (NI-10 80 In the case where the decomposition temperature utilized in zone 14 isgreater than the melting point of sulfur, the bottom water stream fromzone 14 will contain a dispersion of liquid sulfur in the aqueousstream, and this mixture can be passed to sulfur recovery zone 19wherein the liquid sulfur can be allowed to settle out and form aseparate liquid sulfur phase. In this last case, the separation of theelemental sulfur from the treated water stream can be performed, ifdesired, within the decomposition zone by allowing the liquid sulfur tocollect at the bottom of this zone and separately drawing off thetreated water stream and a liquid sulfur stream. This last mode ofoperation is facilitated by the relatively rapid rate that liquid sulfurwill separate from the water stream. In the attached drawing, theseparation of sulfur is accomplished in sulfur recovery zone 19 to whichthe bottom stream from zone 14 is charged via line 18 and from which asulfur stream is with-drawn via line 20 and a water stream is recoveredvia line 21. This last stream is substantially free of ammonia andhydrogen sulfide because of the stripping conditions it experienced inzone 14. Likewise, it is substantially free of sulfide salts (generallysulfide is less than 1,000 ppm. of this stream) and contains only arelatively minor amount of ammonium thiosulfate; typically, about 0.1 toabout 20 wt. percent ammonium thiosulfate calculated as elementalsulfur, the exact value within this range being dependent on the amountof thiosulfate removed from the system via line 27.

A first portion of the water stream from zone 19 is charged via lines 21and 22 to second scrubbing zone 10 where it counter-currently contactsthe gas stream from gas-liquid separating zone 8 which enters zone 10via line 9. Zone 10 is typically a vertically positioned towercontaining suitable contacting means for achieving intimate contactbetween the gas and liquid streams. This zone is usually operated at atemperature which is relativley lower than that used in separation zone8. Likewise, the pressure used is a relatively low pressurecorresponding to that utilized in zone 8 and is preferably about 1 toabout 10 psig. Normally, intimate contact between the gas stream and theliquid stream is effected at a liquid to gas loading sufficient toproduce an overhead stream which is substantially free of NH and H 8. Anitrogen-rich overhead gas stream exits from zone 10 near the topthereof via line 24, and is vented from the system. Similarly, anaqueous bottom stream containing NI-IJIS, NH OH, and (NI-I,,) S O iswithdrawn near the bottom of the zone 10 via line 11. The aqueous streamwithdrawn via line 11 contains substantially all of the hydrogen sulfidean ammonia which is flashed off in separating zone 8.

In the third scrubbing zone, zone 16, the overhead vapor stream frompolysulfide decomposition zone 14, which is charged to the lower regionof zone 16 via line 15, is countercurrently contacted with an ammoniacalwater stream formed by condensing a portion of the overhead vapor fromzone 16 and returning a portion of the resulting condensate to zone 16as reflux. Zone 16 is preferably operated at a pressure corresponding tothat maintained at the outlet from zone 14; for example, good resultsare obtained at a pressure of about 30 to about psig. Similarly, thisthird scrubbing zone is preferably operated at a bottom temperaturewhich is lower than that maintained at the top of zone 14; for example,a temperature of about 250 to about 350 F. Zone 16 is also operated at aliquid gas loading sufficient to produce an overhead vapor streamcomtaining NH, and H 0 which is substantially free of sulfide, and anaqueous bottom stream containing Nl-LOI-I and NH HS. The overhead vaporstream is withdrawn from zone 16 via line 25; thereafter, at least aportion of it is condensed by suitable condensing means (not shown) toform a substantially sulfide-free ammoniacal aqueous stream, and a firstportion of the resulting liquid condensate stream is passed to thirdscrubbing zone via lines 26 and 23 as reflux. The temperature of thisreflux stream should be relatively low as compared to the temperaturemaintained in the bottom of zone 16 for example, where the bottom ofzone 16 is maintained at 300 F. and a pressure of about 60 psig., a goodreflux temperature is 100 F. The aqueous bottom stream from zone 16 iswithdrawn therefrom via line 17. It is to be noted that in the casewhere the gas stream entering the process via line 1 contains NH aportion of the overhead vapor stream from zone 16 can be withdrawn fromthe system via line 25 in order to prevent the buildup of NH; in thesystem. This drag stream is preferably recovered after a portion of theoverhead vapor in line 25 is condensed because subjecting this vaporstream to a partial condensation step acts to purify the NH; vaporstream recovered via line 25.

The second portion of the ammoniacal water stream formed by condensingat least a portion of the overhead vapor stream from zone 16 is thenpassed via line 26 back to the first scrubbing zone as thefirst recyclewater stream. In the case where NH; is not present in the input gasstream to zone 2, line 25 is blocked-off and essentially all of theoverhead vapor stream zone 16 is condensed and used as reflux or as thefirst recycle water stream. In this last case, it is necessary to addmakeup NH or NH OH via line 29 to compensate for the inevitable smalllosses of NH from the system in vent gas streams and in by-products.

A second portion of the water stream recovered from the sulfurseparating step performed in zone 19 is then passed via line 21 to theupperregion of zone 2 as the second recycle water stream. As previouslyexplained, the principal function of this second recycle stream is toscrub NH, from the treated gas stream. Of course it is understood that athird portion of the water stream recovered from zone 19 may bewithdrawn from the system via line 27, either on a continuous orintermittent basis, in order to remove the net amount of (NI- 92820formed in the process.

The thirdportion of the water stream recovered from thesulfur-separation step occurring in zone 19, which is withdrawn from thesystem via line 27, will also contain at least a portion of the waterformed by the oxidation reaction occurring in zone 6. In the case wherethe input gas stream, entering the system via line 1, and/or the inputair stream, which enters via line 5, are not saturated with water, aportion of the water product will be withdrawn from the system via lines3 and 24. Likewise, some water may be withdrawn via line 25 is an NH,product stream is made. In some cases, H,O losses via the vent gasstream and the (NH ),S,O, drag stream may be greater than the waterproduced in the oxidation step; in this case makeup water is added tothe system via line 28. I

In this connection it should be noted that during startup of the presentprocess, a supply of ammoniacal solution is introduced into'the systemvia line 29 and a scrubbing zones and 16, these streams are combinedsupply of sulfide and ammonia-free water via line 28.

After the system is linedout these two input ports are then used tosupply makeup solutions in the manner decomposition step and the thirdscrubbing step is to locate zone 16 on top of zone 14 in piggy-back" atthe junction of line 17 with line 11 to form the third recycle waterstream. The resulting mixture is cooled by a suitable cooling means (notshown) to a temperature of about 50 to about 125 F. and passed via lines11 and 12 back to treatment zone 6 as previously explained. The purposeof the two scrubbing operations is to recapture the unreacted sulfidecontained in the effluent from the, oxidation step in order to preventpollution problems that could be caused by the indiscriminate disposalof this unreacted sulfide, and to increase the yield of elemental sulfurrecovered via line Typical results obtained with the respect process are(l) a conversion of sulfide in zone 6 of about 60 to 90 percent at aselectivity of elemental sulfur at to 99% (on a per pass basis), an (2)an overall conversion of about to about 99 percent of the enteringhydrogen sulfide to elemental sulfur recovered via line 20.

It is intended to cover by the following claims, all changes andmodifications of the above disclosure of the present invention whichwouldbe self-evident to a man of ordinary skill in the gas treating art.7

I claim as my invention:

1. A continuous closed-loop process for scrubbing H S from a gas streamcontaining same and for producing elemental sulfur therefrom, saidprocess comprising the steps of:

l. introducing said gas stream into the lower region of a first gasscrubbing zone, introducing a first recycle water stream containingNH,OH into the middle region of the first scrubbing zone, andintroducing a second recycle water stream which is substantially free ofammonia and sulfideinto the first scrubbing zone at a point above thepoint of introduction of said first recycle stream;

maintaining said first scrubbing zone under counter-current gas-liquidcontact conditions selected to produce a gaseous overhead stream whichis substantially reduced in H,S content and is substantially free ofNH,, and an aqueous bottom stream containing NH HS; Y

. reacting at least a portion of the aqueous bottom stream from step(2), and air stream, and a third recycle water stream containing (NH),S,O,, NH OH, and NH HS with a solid catalyst at oxidizing conditionsselected to form an effluent stream containing ammonium polysulfide,(Nl-l ),S,O,, NH,OH, H,O and N,;

. separating the effluent stream from step (3) into a gas streamcontaining N,, H,O, H,S, and NH,, and a liquid stream containingammonium polysulfide, NH OH, 11,0 and (NH ),S,O,;

. subjecting the liquid stream from step (4) to polysulfidedecomposition conditions effective to produce an overhead vapor streamcontaining NH H,S, and H 0, and an aqueous bottom stream containingelemental sulfur and (NH ),S, s? 1 6. separating sulfur from the bottomstream from step to form a water stream containing a minor amount of(NI- 0 8 0 which stream is substantially free of ammonia and sulfide;

7 contacting a first portion of the water stream from step (6) with thegas stream from step (4), in a second scrubbing zone, at countercurrentgasliquid contact conditions selected to form a nitrogen-rich overheadgas stream and an aqueous bottom stream containing Nl-LOH, (NH S O andNH HS;

8. introducing the overhead vapor stream from step '(5) into a thirdscrubbing zone and operating the third scrubbing zone under refluxconditions selected to form a substantially sulfide-free overhead vaporstream containing NH, and H 0 and an aqueous bottom stream containingNl-LHS and NH OH;

9. condensing at least a portion of the overhead vapor stream from step(8) to form substantially sulfide-free ammoniacal water steam andreturning a first portion of the resulting water stream to step (8) asreflux;

10. combining the bottom stream from step (7) and the bottom stream fromstep (8) to form said third recycle water stream and passing same tostep (3);

remove the net amount of (NI-l ),S 0 formed in the process; and 13.passing a second poriton of the ammoniacal water stream formed in step(9) to step (l) as the first recycle water stream.

2. A process as defined in claim 1 wherein the gas stream charged tostep (1) contains H 8 and NH and wherein a portion of the overhead vaporstream from step (8) is withdrawn fromthe process as an ammoniacalproduct stream.

3. A process as defined in claim 1 wherein the solid catalyst utilizedin step (3) is a phthalocyanine compound.

4. A process as defined in claim 1 wherein the solid catalyst utilizedin step (3) :comprises an iron group metallic sulfide combined with acarrier material.

5. A process as defined in claim 1 wherein the amount of air charged tostep (3) is sufficient to react about 0.4 moles of oxygen per mole ofsulfide charged to said step.

6. A process as defined in claim 1 wherein said solid catalyst utilizedin step (3) is cobalt phthalocyanine monosulfonate combined with anactivated carbon carrier material.

7. A process as defined in claim 1 wherein said gas stream charged tostep (I is a coal gas containing H 8.

8. A process as defined in claim 1 wherein said gas stream charged tostep (1) is a water gas or a synthesis gas containing H 8.

9. A process as defined in claim 1 wherein said gas stream charged tostep (1) is a natural gas containing H28- 10. A process as defined inclaim 1 wherein said gas stream charged to step (1) is a refinery gascontaining l l. A process as defined as in claim 1 wherein the oxidizingconditions utilized in step (3) include a temperature of about to about200 F. and a pressure of about 1 to about 30 psig.

2. A process as defined in claim 1 wherein the gas stream charged tostep (1) contains H2S and NH3 and wherein a portion of the overheadvapor stream from step (8) is withdrawn from the process as anammoniacal product stream.
 2. maintaining said first scrubbing zoneunder counter-current gas-liquid contact conditions selected to producea gaseous overhead stream which is substantially reduced in H2S contentand is substantially free of NH3, and an aqueous bottom streamcontaining NH4HS;
 3. reacting at least a portion of the aqueous bottomstream from step (2), and air stream, and a third recycle water streamcontaining (NH4)2S2O3, NH4OH, and NH4HS with a solid catalyst atoxidizing conditions selected to form an effluent stream containingammonium polysulfide, (NH4)2S2O3, NH4OH, H2O and N2;
 3. A process asdefined in claim 1 wherein the solid catalyst utilized in step (3) is aphthalocyanine compound.
 4. A process as defined in claim 1 wherein thesolid catalyst utilized in step (3) comprises an iron group metallicsulfide combined with a carrier material.
 4. separating the effluentstream from step (3) into a gas stream containing N2, H2O, H2S, and NH3,and a liquid stream containing ammonium polysulfide, NH4OH, H2O and(NH4)2S2O3;
 5. subjecting the liquid stream from step (4) to polysulfidedecoMposition conditions effective to produce an overhead vapor streamcontaining NH3, H2S, and H2O, and an aqueous bottom stream containingelemental sulfur and (NH4)2S2O3;
 5. A process as defined in claim 1wherein the amount of air charged to step (3) is sufficient to reactabout 0.4 moles of oxygen per mole of sulfide charged to said step.
 6. Aprocess as defined in claim 1 wherein said solid catalyst utilized instep (3) is cobalt phthalocyanine monosulfonate combined with anactivated carbon carrier material.
 6. separating sulfur from the bottomstream from step (5) to form a water stream containing a minor amount of(NH4)2S2O3, which stream is substantially free of ammonia and sulfide; 7contacting a first portion of the water stream from step (6) with thegas stream from step (4), in a second scrubbing zone, at countercurrentgas-liquid contact conditions selected to form a nitrogen-rich overheadgas stream and an aqueous bottom stream containing NH4OH, (NH4)2S2O3,and NH4HS;
 7. A process as defined in claim 1 wherein said gas streamcharged to step (1) is a coal gas containing H2S.
 8. A process asdefined in claim 1 wherein said gas stream charged to step (1) is awater gas or a synthesis gas containing H2S.
 8. introducing the overheadvapor stream from step (5) into a third scrubbing zone and operating thethird scrubbing zone under reflux conditions selected to form asubstantially sulfide-free overhead vapor stream containing NH3 and H2Oand an aqueous bottom stream containing NH4HS and NH4OH;
 9. condensingat least a portion of the overhead vapor stream from step (8) to formsubstantially sulfide-free ammoniacal water steam and returning a firstportion of the resulting water stream to step (8) as reflux;
 9. Aprocess as defined in claim 1 wherein said gas stream charged to step(1) is a natural gas containing H2S.
 10. A process as defined in claim 1wherein said gas stream charged to step (1) is a refinery gas containingH2S.
 10. combining the bottom stream from step (7) and the bottom streamfrom step (8) to form said third recycle water stream and passing sameto step (3);
 11. recovering a second portion of the water stream fromstep (6) as said second recycle water stream and passing same to step(1);
 11. A process as defined as in claim 1 wherein the oxidizingconditions utilized in step (3) include a temperature of about 75* toabout 200* F. and a pressure of about 1 to about 30 psig. 12.withdrawing a third portion of the water stream produced in step (6)from the process in order to remove the net amount of (NH4)2S2 O3 formedin the process; and
 13. passing a second poriton of the ammoniacal waterstream formed in step (9) to step (1) as the first recycle water stream.